Fix Spree::Api:LineItemsController#create handling validation errors

Before, #create was expecting that the LineItem instance would have
errors if failed to be created due to validation errors. However,
 #add actually raises ActiveRecord::RecordInvalid: Validation failed,
so this endpoint could have never handled validation errors correctly.

api/app/controllers/spree/api/line_items_controller.rb

@@ -11,16 +11,15 @@ def new
 
       def create
         variant = Spree::Variant.find(params[:line_item][:variant_id])
-        @line_item = @order.contents.add(
-          variant,
-          params[:line_item][:quantity] || 1,
-          options: line_item_params[:options].to_h
-        )
-
-        if @line_item.errors.empty?
+        begin
+          @line_item = @order.contents.add(
+            variant,
+            params[:line_item][:quantity] || 1,
+            options: line_item_params[:options].to_h
+          )
           respond_with(@line_item, status: 201, default_template: :show)
-        else
-          invalid_resource!(@line_item)
+        rescue ActiveRecord::RecordInvalid => error
+          invalid_resource!(error.record)
         end
       end
 

api/spec/requests/spree/api/line_items_spec.rb

@@ -95,6 +95,12 @@ module Spree::Api
         expect(response.status).to eq(201)
       end
 
+      it '#create calls #invalid_resource! if adding a line item fails validation' do
+        allow_any_instance_of(Spree::LineItem).to receive(:valid?).and_return(false)
+        expect_any_instance_of(Spree::Api::BaseController).to receive(:invalid_resource!).once
+        post spree.api_order_line_items_path(order), params: { line_item: { variant_id: product.master.to_param, quantity: 1 } }
+      end
+
       it "default quantity to 1 if none is given" do
         post spree.api_order_line_items_path(order), params: { line_item: { variant_id: product.master.to_param } }
         expect(response.status).to eq(201)